Semiconductor devices operate by placing a plurality of regions of differently doped semiconductive material adjacent one another. The electrical proficiency of the device is strongly affected by the purity of the semiconductor materials used in fabricating the device. This is especially true as integrated circuits become smaller. Contaminants take up a greater percentage of real estate in smaller devices, leaving less room for desirable atoms. Thus, current is more critically limited by the purity of semiconductor materials in smaller devices.
Accordingly, the number of contaminating particles which adhere to the device during processing must be carefully controlled. Integrated circuits comprising semiconductor devices are typically fabricated in a clean room. Clean rooms are rooms which have strict controls as to the number of particles, such as dust, that are allowed to exist in the atmosphere of the room. However, it is impossible to ensure that no contaminating particles are present in a clean room. Even the highest grade clean room has particles present in the atmosphere, due to human operator presence and machinery for example, which can contaminate a wafer during processing.
Typically, semiconductor devices are processed in baths, such as etching solutions or cleaning solutions, left open to the atmosphere of the clean room. Dust particles, inherent even in clean rooms, settle on the surface of the bath liquid. As a wafer is inserted into or removed from a bath, the wafer is exposed to numerous particles settled on the surface of the bath. Thus, particles become attached to the semiconductor wafer surfaces during wet processing. Avoidance of contamination at the bath surface requires that a wafer be quickly inserted into or removed from a bath so that the wafer surfaces are not exposed to contaminants at the liquid/air interface for long periods of time.
Additionally imposing constraints on the time a wafer spends in a bath is the possible detrimental effect of the bath liquid on the surface of the wafer. For example, if a semiconductor is exposed to an etching solution for too long, too much of the semiconductor material may be etched away. Thus, the duration of a wafer's exposure to processing chemicals must be critically controlled.
Several instruments have been employed to purify wafers during processing which are germane to the present invention. One instrument, a megasonic energy cleaning apparatus, has been advantageously employed to clean contaminants from semiconductor surfaces. It is disclosed in U.S. Pat. No. 4,869,278 to Bran. Megasonic energy cleaning apparatuses comprise a piezo-electric transducer adhered to a transmitter. The transducer is electrically excited such that it vibrates. In combination with the transmitter, high frequency energy is emitted into a tank containing liquid, thereby vibrating the liquid in the tank.
When used in a semiconductor wafer wet processing tank, the vibrational energy is directed over the surfaces of semiconductor wafers. Contaminants are thus vibrated away from the surfaces of the wafer. When the wafer is removed, the surfaces are cleaner than if the wafer was merely inserted into a stationary bath of processing fluid.
Another device utilized to purify semiconductor wafers is disclosed in U.S. Pat. Nos. 4,633,893, 4,795,497, 4,856,544, and 4,899,767 to McConnell et al. In this device fluids flow continuously over the faces of the wafers, allowing the wafers to remain stationary and constantly submerged in liquid. Thus, during processing, no machinery is necessary to move the wafers from one bath to another and the wafers are not exposed to the atmosphere of the clean room. However, the bath liquid merely flows over the wafer surface, removing loose contaminants. If a contaminating particle is substantially stuck to the surface of the wafer, this system may not remove it.
It is clear from the above discussion that semiconductor wafer processing requires considerable supervision. Thus, batch processing, or the processing of a plurality of wafers simultaneously, is generally considered the most cost effective means. Batch processing typically increases the number of wafers processed per unit time with respect to the number of wafers that could be completed if a single wafer at a time were processed.
However, batch processing has many drawbacks. By processing a plurality of semiconductor wafers at the same time, contaminants from each wafer are released into the processing liquids. Additionally, if a cassette holding a plurality of wafers is accidently harmed during processing, by being dropped for example, each of the wafers is damaged, possibly beyond usefulness. As purity requirements are raised, they become increasingly difficult to achieve, particularly with an entire batch. Also, as wafers become larger and more costly, the processing of a large batch becomes riskier.